CDC25 family dual-specific protein phosphatases activate cyclin-dependent kinases (CDKs) by dephosphorylation of the ATP binding domains of the kinases, functioning as rate-limiting regulators of eukaryotic cell cycle progression. In response to DNA damages, CDC25 phosphatases are inactivated by CHK1/2-dependent phosphorylation, forming critical targets of cell cycle checkpoint. Of mammalian three CDC25 proteins, CDC25A appears to play a distinct and indispensable role in developmental and checkpoint control of the cell cycle, while the functions of CDC25B and CDC25C in development and checkpoint appear more overlapping or dispensable. CDC25A is overexpressed in a variety of human cancer tissues, including breast cancer, although cancer-associated amplification of the CDC25A locus is not usually observed. The long-term goal of this continued research program is to understand the exact role of CDC25A in breast cancer and establish a scientific basis for anti-cancer therapies targeted on this protein. Previous studies indicate that CDC25A overexpression, which is observed in a population of early stage breast cancers, correlates with negative expression of estrogen and progesterone receptors, p53 mutations, high tumor grades and poor prognosis of patients. The central hypothesis evaluated in the present application is that p53 inactivation and deregulated CDC25A expression promote each other with a positive feedback mechanism, and both events functionally cooperate in establishing chromosome instability during tumor initiation and exacerbating malignant phenotypes, especially in triple-negative breast cancer. In the renewed research program, three specific aims will be pursued: (1) Determine how deregulated CDC25A expression cooperates with p53 inactivation in the development of breast cancer; (2) Determine the mechanisms and significance of chromosome instability in mammary epithelial cells with deregulated CDC25A expression; (3) Define in vivo interactions of CDC25A with p53-dependent checkpoint and apoptosis during mammary tumorigenesis. These studies should provide significant insight into the oncogenic roles of CDC25A, which is now regarded as a therapeutic target.